This invention relates to a process for treating aqueous polymer solutions used in polymer flooding. The aqueous polymer solutions are treated under turbulent flow conditions at a specific pump energy and residence time. 2. Description of the Prior Art
Polymer flooding is a known method of tertiary oil recovery as detailed, for example, in Nachr. Chem. Tech. Lab. 27 (1979), No. 6. The use of water-soluble polymers in the flooding water lowers the mobility of the flooding medium, thus improving the effectiveness of flooding per unit volume. Compared to conventional water flooding, larger areas of the deposit can be treated and thus the oil can be recovered from larger areas, which significantly increases the degree of recovery for a deposit.
Most polymer flooding projects use partially saponified polyacrylamides, since these polymers exhibit the most pronounced viscosity-increasing characteristics in lowsaline waters. Preferably, so-called polymer emulsions are used at the start. Droplets of water are dispersed in an organic phase, which contains the polymer. After phaseinversion, the swollen polymer particles can then quickly dissolve in the aqueous phase, which is then continuous.
ln order to optimize the flow properties in the deposit, the polymer solutions must meet a range of specifications. These specifications can then be shifted to match them to the type of conditions encountered in the deposit. In addition to chemical and biological long-term stability as well as insensitivity to outside ions, characteristics which will not be discussed further here, the following properties are of importance:
1. Minimum Pressure Increase in the Filtration Test PA1 2. Optimum Size of the Residual-Residence Factor (RRF) PA1 3. Linearity of the Resistance Factor (RF) in the Minimum velocity Range (V.sub.D &lt;0.1 m/d) PA1 4. Minimal Increase in the Resistance Factor in the Injection Velocity Range (10 m/d&lt;V.sub.D &lt;100 m/d) PA1 5. High Effective Viscosity in Pore Flow in the Medium Deposit Velocity Range (0.1 m/d&lt;V.sub.D &lt;100 m/d)
Although filtration tests using Millipore filters only approximately describe the flow processes in deposit rock, this test method has become standard (Kohler, N .G. Chauvetau, SPE 7435, 1978). The increase in pressure during the filtration test should be kept as low as possible.
The RRF is a factor indicating the extent to which the permeability of the rock has been reduced after polymer flooding. The optimum RRF for a polymer solution depends on the given deposit. As a rule, it is determined by simulation calculations. This means that it is necessary to match the RRF to deposit conditions.
At the low flow velocities which occur in zones of extremely low permeability, the RF factor must not increase as in the medium-velocity range. Instead, it should assume a constant value, since otherwise it becomes even more difficult to flood the low permeability zones.
Based on viscoelastic properties, solutions of partially saponified polyacrylamides exhibit an undesired increase in their RF values in the range of typical injection velocities (Seright, R. S., SPE 9297, 1980). This makes it necessary to use an increased injection pressure, which often cannot be done because of the mechanical properties of the rock, since there would be danger of hydraulic cracking. ln order to be able to inject sufficient amount of the polymer solution into the deposit, the resistance factor must not increase in the injection velocity range, or it must only increase slightly.
In order to maintain a pronounced mobility-reducing effect in the medium deposit velocity range, the effective viscosity of the polymer solutions in pore flows should be as high as possible and the measures required to improve the properties listed in sections 1 through 4 above should only be reduced slightly.
It is known that dissolved, partially saponified polyacrylamides may be broken down when subjected to mechanical loads. Thus, it is not recommended that centrifugal pumps be used when these solutions are to be pumped (D. Norgan, Oil and Gas Journal, 54: 1976). Since the higher molecular weight components are destroyed under mechanical stress, and since this adversely affects the properties cited in sections 1 through 4, the prior art teaches that the polymer solutions are subjected to controlled mechanical stress other than that caused by centrifugal pumps, in order to improve the properties cited under sections 1 through 4.
lt is also known that flooding rock material typical of the site at sufficiently high flow rates and reusing the solutions improves the RF curve in the injection velocity range (R.S. Seright, SPE 9297, 1980). However, this observation cannot be utilized in the field for practical reasons. Subjecting the polymer solutions to subsequent shear through a perforated plate in accordance with Federal Republic of Germany Patent document 27 33 852 also improves rheology. According to this patent, the drop in pressure at the same flow rate remains constant for a given permeability and porosity of a rock pore or sand packing (the permeability and porosity are not described further). In addition, the curve of shear viscosity as a function of the shear rate is improved, and the injection pressure for flooding a sand packing is reduced from 0.38 bar to 0.16 bar, whereby the shear viscosity at a shear rate of 100 s.sup.-1 is only reduced from 77.5 mPas to 7.3 mPas. It is also known from the Federal Republic of Germany Patent 33 03 895 that the filterability of a heteropolymeric sucrose solution can be improved through shear treatment in a colloid mill.
Thus, so far no process has become known which both optimizes the properties listed in sections 1 through 4 and meets the requirements cited in section 5.